CD4+ T helper cells are central regulators of adaptive immune responses. CD4+ T helper cells differentiate into several distinct subsets to provide host protection against a variety of pathogens. Each T helper cell lineage expresses characteristic transcription factors and cytokines that confer specific effector functions. The cytokine-secreting potential of effector T helper subsets requires the activation and expression of transcription factors that promote the development of each subset. Differentiation is stimulated by the cytokine microenvironment and the activation of Signal Transducer and Activator of Transcription (STAT) proteins that initiate specific genetic programs. A simple paradigm suggested the requirement for a single STAT protein activated by a cytokine that promoted the development of a specific T helper subset. However, developing Th subsets are exposed to multiple cytokines. We have recently provided evidence that the ability of Th cells to integrate signals from multiple cytokines is necessary for optimal subset development. Specifically, we demonstrated that STAT3, which clearly promotes Th17 development in isolation, promotes Th2 differentiation when it cooperates with STAT6 in programming Th2 cytokine expression by binding to many of the same loci as STAT6. In this proposal we test the hypothesis that STAT3 binds to common and unique genes among Th subsets by using chromatin immunoprecipitation and massive parallel sequencing to define the targets of STAT3 in multiple T cell subsets. These studies will define how a developing Th cell integrates multiple signals at the level of transcription factor binding and provide the basis for a new paradigm wherein a single STAT may be the key to a particular phenotype, whereas other STAT proteins cooperate to achieve optimal differentiation and ultimately immune responses.